Projection imaging systems are widely used in photolithographic steps of semiconductor fabrication and in laser machining of metals, ceramics and polyimide. A conventional projection system includes a light source, a mask consisting of transmitting and blocking regions, and projection optics. When the light source is a laser, the additional element of a diffuser is included to make the light spatially incoherent to minimize the occurrence of standing waves resulting from interference fringes. Diffusers can typically be a fly's eye beam scrambler or a cloud-like scattering element, and must be capable of handling high power illumination.
Imaging system masks modulate light by selectively reflecting and/or absorbing the light at the blocking regions. Three commonly used mask types are metal, chrome on glass and dielectric.
Metal masks are formed by cutting or etching holes in sheet metal. The blocking mechanism is partial reflection and absorption of the incident light. At high powers and/or extended exposures, absorption of light heats the mask and distorts it from its intended shape. As a result, metal masks for use with high power lasers generally require active cooling.
Chrome-coated glass masks use reflection as the blocking mechanism. However, enough incident light is still absorbed by the blocking portion that, especially at ultraviolet wavelengths, over time the thin chrome film erodes, generating mask defects. It is known that bridged areas on chrome masks can be corrected by burning off the defect with a laser, so it is clear that chrome masks are severely limited in their use with high power lasers of at least certain wavelengths.
Dielectric masks use reflection as a blocking mechanism and exhibit negligible absorption. However, for pulsed operation in the ultraviolet range, the damage threshold of a dielectric mask is typically 1-3 Joules/cm.sup.2 due to the fragile nature of the reflective coating on the mask.
From diffraction theory, it is known that binary phase plates will modify the wavefront of incident light which is coherent and has a narrow bandwidth by using steps proportional to one-half the wavelength of the incident light to advance or retard phase or to scatter the light exiting from the phase plate.
Phase plates are relatively easy to manufacture in that common lithographic techniques can be used. Phase plates also have the advantage that, since they are diffractive elements, there is no significant absorption and, therefore, no heat-induced distortion or erosion of the pattern. Since they do not rely on reflection of any kind, optical coatings are not required. Thus, a phase mask possesses none of the negative qualities of the above-described mask types.
It would be desirable to provide a mask for use in high power laser processes which possesses the advantages of phase plates while still allowing the same or better masking of the pattern to be projected upon the workpiece as can be accomplished using conventional masking techniques. It is to such an objective that the present invention is directed.